Adjusting the element to the grid

How to correctly adjust the dynamic dimensions of an element to existing window dimensions?

Situation:

• grid occupies some part of the entire window
• in the middle, we draw an element, where we give the height and width of the elements and can be other dimensions
• I would like to keep the element filling the maximum indicated field while maintaining the ratio of dimensions

Previously, I was doing functions that took parent dimensions and each dimension analyzed by a separate function, but this dramatically slows down graphics rendering